A. Field of the Invention
The present invention is directed to a duct averaging sensor for detecting an environmental condition of air within a duct. The duct averaging sensor is particularly suited to sensing the temperature of air as the air passes through the duct.
B. Statement of the Related Art
In a large heating, ventilation and air conditioning (HVAC) system, an air handling unit (AHU) may heat, cool, filter or humidify air to condition the air for the comfort of the occupants of a building. The AHU moves the conditioned air, known as supply air, through supply ducts and into the occupied spaces of the building. Return ducts transport return air from the occupied spaces back to the AHU. Makeup air from outside the building is mixed with the return air and the AHU conditions the combination of the return air and makeup air, completing the loop.
A control system is informed by sensors and controls the AHU. Sensors may be located within the return or supply ducts or both to monitor the return air and supply air. The control system may include other sensors as well. A sensor includes a sensor element and detects an environmental condition. As used in this document, the term ‘environmental condition’ means a characteristic of the air within the duct, for example: temperature, humidity and any other condition that may be useful to control the AHU. As used in this document, the term ‘sensor element’ refers to an apparatus capable of detecting an environmental condition at one location.
A sensor may utilize a single sensor element and detect an environmental condition at a single location within a duct. The weakness of a sensor utilizing a single sensor element is that the air in the duct may be stratified, with air at different locations in a cross section of the duct having different characteristics. Since the single sensor element can detect the environmental condition at only a single location, the single sensor element will provide the control system with incomplete information, resulting in poor control.
Duct averaging sensors address the shortcomings of sensors having a single sensor element by providing multiple sensor elements that detect the condition of the air at multiple locations within the duct. The multiple sensor elements are located in a single housing, which may be a length of tube. The multiple sensor elements are distributed along the length of the housing and detect the condition of the passing air at multiple locations along the length of the housing. For a large duct, the housing may be elongated, for example six, twelve or twenty-four feet in length, and may be a bendable tube composed of soft copper or aluminum. Such a duct averaging sensor features multiple sensor elements, for example, nine sensor elements. The multiple sensor elements are electrically connected to each other and terminate in a junction box permanently attached to one end of the tubing. As an alternative to a bendable housing, the housing may be straight and rigid.
In prior art duct averaging sensors, installation of the housing containing the sensor elements presents challenges to the installing technician, making proper installation a two-person job. As received by the technician, the bendable housing of a prior art duct averaging sensor is coiled and one end of the housing is permanently attached to the junction box. To install the prior art duct averaging sensor, a first technician drills a hole through the wall of the duct from the outside of the duct. The first technician then un-coils the coiled housing and passes the free end of the housing through the drilled hole from the outside of the duct to the inside of the duct. The first technician continues to pass the length of the housing through the hole until the junction box contacts the outside of the wall of the duct. A second technician located inside of the duct receives the free end of the housing and protects the housing as its entire length passes through the drilled hole and into the duct. The second technician arranges and secures the housing within the duct. The second technician arranges the bendable housing to locate the sensor elements to detect the condition of the air in the desired locations within the duct. The first technician attaches the junction box to the outside of the wall of the duct and connects the electrical leads of the duct averaging sensor to the control system. The electrical connections generally are made using screw terminals, wire nuts or solder.
The step of passing the entire length of the housing through the drilled hole presents a hazard of flexing or kinking the housing. Flexing or kinking the housing can damage the sensors, wires or connections contained within the housing. An apparatus for quickly and reliably installing a duct averaging sensor is needed.